Carbokentbrooksite Explained

Carbokentbrooksite
Category:	Silicate mineral, Cyclosilicate
Formula:	(Na,□)12(Na,Ce)3Ca6Mn3Zr3Nb(Si25O73)(OH)3(CO3)·H2O (original form)
Imasymbol:	Cktb^[1]
Strunz:	9.CO.10 (10 ed) 8/E.25-32 (8 ed)
Dana:	64.1.2.3
Class:	Ditrigonal pyramidal (3m) (same H-M symbol)
Symmetry:	R3m
Unit Cell:	a = 14.24, c = 30.04 [Å]; Z = 3
Color:	Yellow, yellow-orange
Habit:	rhombohedra (cores of zoned crystals)
Cleavage:	None
Fracture:	Conchoidal
Tenacity:	Brittle
Mohs:	5
Luster:	Vitreous
Streak:	White
Diaphaneity:	Transparent
Density:	3.14 (measured)
Opticalprop:	Uniaxial (−)
Refractive:	nω = 1.65, nε = 1.64 (approximated)
Pleochroism:	None
Fluorescence:	No

Carbokentbrooksite is a very rare mineral of the eudialyte group,^[2] with formula (Na,□)₁₂(Na,Ce)₃Ca₆Mn₃Zr₃NbSiO(Si₉O₂₇)₂(Si₃O₉)₂(OH)₃(CO₃)^.H₂O.^[3] The original formula was extended to show the presence of cyclic silicate groups and silicon at the M4 site, according to the nomenclature of eudialyte group.^[4] Carbokenbrooksite characterizes in being carbonate-rich (the other eudialyte-group species with essential carbonate are zirsilite-(Ce), golyshevite, and mogovidite). It is also sodium rich, being sodium equivalent of zirsilite-(Ce), with which it is intimately associated.

Occurrence and association

Carbokentbrooksite and zirsilite-(Ce) are found as replacements of grains and crystals of eudialyte. They occur in pegmatites of Darai-Pioz alkaline massif, Tajikistan – a locality known for many rare minerals.^[5] The minerals are associated with aegirine, ekanite, microcline, polylithionite, quartz, stillwellite-(Ce) (silicates), pyrochlore-group mineral, fluorite, calcite, and galena.

Notes on chemistry

Beside the elements given in the formula, carbokentbrooksite contains admixtures of lanthanum, strontium, neodymium, iron, yttrium, titanium, potassium, chlorine, and praseodymium. Carbokentbrooksite and zirsilite-(Ce) are chemically similar.

Notes on structure

Carbokentbrooksite is isostructural with kentbrooksite.

Notes and References

1. Warr. L.N.. 2021. IMA–CNMNC approved mineral symbols. Mineralogical Magazine. 85. 3. 291–320. 10.1180/mgm.2021.43. 2021MinM...85..291W. 235729616. free.
2. Mindat, Carbokentbrooksite, http://www.mindat.org/min-25674.html
3. Khomyakov, A.P., Dusmatov, V.D., Ferraris, G., Gula, A., Ivaldi, G., and Nechelyustov, G.N., 2003: Zirsilite-(Ce), (Na,[])₁₂(Ce,Na)₃Ca₆Mn₃Zr₃Nb(Si₂₅O₇₃)(OH)₃(CO₃)•H2O, and carbokentbrooksite (Na,[])₁₂(Na,Ce)₃Ca₆Mn₃Zr₃Nb(Si₂₅O₇₃)(OH)₃(CO₃)•H₂O – two new eudialyte-group minerals from the Dara-i-Pioz alkaline massif, Tajikistan. Zapiski Vserossiyskogo Mineralogicheskogo Obshchestva 132(5), 40–51 (in Russian, with English abstract); in: Jambor, J.I, and Roberts, A.C., 2004: New mineral names. American Mineralogist 89(11–12), 1826–1834
4. Johnsen, O., Ferraris, G., Gault, R.A., Grice, D.G., Kampf, A.R., and Pekov, I.V., 2003. The nomenclature of eudialyte-group minerals. The Canadian Mineralogist 41, 785–794
5. Web site: Darai-Pioz Glacier (Dara-i-Pioz; Dara-Pioz), Alai Range (Alayskiy), Tien Shan Mtn, Region of Republican Subordination, Tajikistan - Mindat.org . Mindat.org . 2016-03-11.